You have either a Marlin Model 17, or a Model 26. The model number should be stamped on the upper tang, behind the hammer. Both had the 1904 patent date, but the Model 17's last patent date is 1904, whereas the Model 26 last patent date was 1906.
Looks like a 20 inch barrel, which Marlin did offer as a Riot Gun. To be sure it is factory, look on the side of the barrel for a serial number (may be behind the pump arm) and a letter. If it is a factory Riot Gun there will be a letter "C" for Cylinder Bore or open choke. If you see the letter "F" for Full Choke or "M" for Modified Choke, then the barrel has been cut later after it left the factory.
The "A" letter serial number prefix indicates it was made in 1916.
Does the butt plate have "Marlin" in script at the toe or is it blank?

Due to the increasing cost of ammunition, there be no warning shot!

The growing federal deficit = generational slavery to the national debt.